In a variety of wellbore drilling operations, drill bits are deployed on a drill string and used to cut through rock formations to create a wellbore. Operation of the drill bit creates cuttings that are removed by using drilling mud flowing downhole to clear the cuttings and to carry the cuttings uphole with the returning drilling mud. The cuttings can be used to obtain many types of information related to the drilling operation and to the subterranean environment.
Sometimes the term “mud-logging” is used to describe the capture and evaluation of cuttings from the drilling operation. Mud-logging comprises the recordation of cuttings lithology and wellbore gases at sequentially measured depths to create a log providing a lithological and gas record of the drilled wellbore. Accurate measurement of the depth at which the cuttings were produced is important for analysis of the drilling operation and subterranean environment. Generally, the depth from which the cuttings were made is calculated based on the volume of the wellbore annulus and the pump stroke rate of the mud pump used to deliver drilling mud. As the drill bit cuts through the rock, cuttings are released into the fluid stream of the flowing mud and subsequently collected at the surface for analysis. Ideally, the cuttings arrive at the surface one annulus volume later as measured by strokes of the mud pumps. The lag-time and knowledge of the annulus volume are used to estimate the depth at which the cuttings were produced.
However, the drilling operation often is conducted through a very dynamic environment with a variety of different processes that can affect the flow of fluid and therefore the transport of cuttings. For example, the flow of fluid and cuttings often can be disrupted which renders the depth determination indicated on the mud log subject to inaccuracies. Additionally, the wellbore can be washed-out and form wellbore sections having a larger gauge than the drill bit gauge. The larger sections change the wellbore annulus volume and again affect the accuracy of the calculated source depth of the cuttings returning to surface.